PROJECT SUMMARY The goal of this project is to identify mechanisms by which the adhesion receptor, PSGL-1 (P-selectin glycoprotein-1) regulates the establishment of a chronic LCMV infection and to assess whether targeting this receptor represents a clinically translational approach to treating chronic viral infections. With more than 500 million people infected with HIV, HCV, and HBV worldwide, chronic viruses represent a major global health problem. The murine model of chronic LCMV infection has been widely demonstrated to have clinical relevance to human chronic infections. Although numerous anti-viral compounds and biologicals can inhibit viral replication, current therapies are associated with significant toxicity and/or immunopathology. It is therefore critically important to develop new targets for treat these infections. Of significance, our studies suggest that ligation PSGL-1 on T cells may be a pivotal event that limits the effector T cell accumulation and leads to dysfunctional virus-specific T cell responses that underlie chronic viral infections. Our data show that the chronic LCMV virus (Clone 13/Cl 13) is unable to establish chronicity in mice that are genetically deficient in PSGL-1, a major receptor for the selectin family of adhesion molecules (P, E, and L). Instead, there are dramatic increases in the numbers of virus-specific CD8+ T cells, deletion of virus-specific T cells is curtailed, and persisting anti-viral T cells fail to develop the exhausted phenotype characterized by sequential loss of effector functions. Virus-specific T cells from PSGL-1 KO mice express greatly reduced levels of immune inhibitory receptors, resulting in enhanced CD8+ T cell functional responses during chronic infection. In addition, the absence of PSGL-1 also supports enhanced virus-specific CD4+ T cell responses to LCMV Cl 13. Most notably, the preservation T cell functionality is associated with viral clearance and severe immunopathology, highlighting a critical function for PSGL-1 as an immune regulator during chronic infection. The data show that there is a broad impact of PSGL-1 on T cell chronic anti-viral responses and support the hypothesis that PSGL-1 is a previously unknown key to negative regulation of T cell immune function. On the basis of our findings we propose to investigate the mechanisms engaged by PSGL-1 to limit anti-viral immunity and explore therapeutic approaches to target this receptor to enhance anti-viral effector T cell responses.